Patent GB-579530-A (GB)
Title: Improvements in or relating to electric measuring apparatus
IPC: G01R17/00
Filed: 1944-05-18
Abstract: 579,530. Valve amplifying circuits. STANDARD TELEPHONES & CABLES, Ltd., and SECKER, B. May 18, 1944, No. 9595. [Class 40 (v)] [Also in Group XXXVI] An impedance measuring bridge arrangement is supplied from an oscillator over terminals 1, 2, Fig. 4, through a transformer having two secondaries S1, S2 forming the ratio arms. The output from the bridge shown as a unit BU, is connected through an amplifier AMP of adjustable gain to two similar modulators MOD.A, MOD.B in series (or parallel). The oscillator supplying the bridge unit BU also supplies over circuit LC a phase changer PC comprising two stages PC1, PC2. Stage PC1 introduces a phase change to compensate for the phase change produced in the bridge output while passing through amplifier AMP. Stage PC2 produces two separate output currents in quadrature which are supplied over lines LA, LB respectively to modulators MOD.A, MOD.B to serve as carrier currents therefor. The modulators are arranged as described in Specification 540,579, whereby the reading of meter M1 in modulator MOD.A is proportional to the in-phase component of the bridge output v and the reading of meter M2 in Mod.B is proportional to the quadrature component. In the amplifier AMP, the grid circuit of V1 may be tuned to the frequency of the oscillator supplying the bridge and gain may be adjusted by a variable negative feedback connection R2 in the cathode resistor of valve V1. The modulators each comprise an input transformer T1, T2, the secondary of which is divided into two equal portions separated by an adjustable potentiometer P1, P2, and a direct current measuring instrument M1, M2 shunted by series connected condensers K1, K2 and connected to the corresponding transformer through a rectifier bridge B1, B2. The phase-changer stage PC1 comprises an adjustable phaseshifting network F1 in the form of a bridge R3, R4, C4, R5, the output from which is connected across the grid-cathode of amplifying valve V4. The output from V4 is connected through transformer T3 to a phase-splitting network F2 in the stage PC2. By adjustment of R6, the voltage across resistance G1 is brought into quadrature with that across G2. These.voltages are amplified respectively by valves V5, V6, the output from which are passed to the modulators through transformers T5, T6 over lines LA, LB. In operation, the switch bank in the bridge unit is moved to position 1 where the input terminals of amplifier AMP are short circuited. Potentiometers P1, P2 are then adjusted until the meters M1, M2 read zero, thereby eliminating carrier leak. The switch is now set to position 2 in which large capacity calibrating condenser C1, C2, having small and equal phase differences from 90 degrees and differing in capacity by a known small amount are switched in the bridge circuit; The resistance R5 in PC1 is now adjusted until the meter M2 reads zero. This ensures that the carrier current supplied to MOD.B is exactly in quadrature with the voltage supplied to'the modulator from the amplifier so that the phase change through the amplifier AMP has been neutralized. The switch is now set-in position 3 which substitutes condenser C3 for C2. C3 has a known phase difference from C1 as indicated by resistance shunt R7, but is of substantially equal capacity. Resistance R2 in the amplifier is now adjusted so that meter M2 reads the known phase difference between C1, C3. Resistance R6 is finally adjusted so that M1 reads zero. This ensures that the two carriers are in exact quadrature. The switch is now moved to position 4 where the test impedance and the comparison impedance are switched in the bridge circuit, when M1 reads off directly the percentage capacity difference and M2 the phase difference. Protective arrangements. Fig. 5 shows a modified arrangement of the amplifier AMP for automatically rendering the modulators inoperative except when the bridge is nearly in balance, so preventing damage to the meters M1, M2. In this case, a double diode triode V14 is employed in addition to the normal three amplification stages V11 ... V13. The triode portion of V14 contains in its anode circuit the main winding m of a polarised relay F, the bias winding n of which is connected in series with a resistance directly across the H.T. source. When the bridge is substantially balanced, a relatively large anode current passes through the triode anode of V14 so that the main winding m of relay F holds the contacts f thereof in the position shown when the amplifier output from V13 is connected over contacts dl of a donkey relay D to the modulators for normal operation thereof. When the bridge gives a large unbalance output, however, a relatively large alternating voltage appears across transformer T7 in the anode circuit of V12 'and is rectified by the diode portions of V14. The grid potential of V14 is thereby reduced and the anode current falls so that the bias winding of relay F takes control and switches over contacts f. This operates relay D which at contacts d1 disconnects the modulator input transformers T1, T2 from the amplifier output, and shunts a substitution resistance R12 across T1, T2 which maintains balance for carrier leak so that both meters M1, M2 now read zero. At the same time contacts d1 connect the anode of V13 directly to its load resistance R10, and contacts d2 close to switch on a warning lamp WL. When the bridge is rebalanced, the relay F does not change back until the bridge unbalance has been reduced to a value less than that at which relay F operates to cut out the modulators. This is effected by a resistance R11 shunted across winding n and controlled by contacts f and prevents chattering of relay F. Separate phase changers may be used in place of PC. The protective arrangement of Fig. 5 may be used with the arrangement of Fig. 4.